Semiconductor laser diodes are essential components for optical amplifier systems of the kind expected to fulfill the increasing requirements of rapid and large capacity communications. In particular, the increasing density of communications channels in dense wavelength division multiplexing (DWDM) operation requires increased power and an ever-tighter degree of control of fiber amplifier gain flatness. This, in turn, relates to the power capacity, wavelength stability and control of the pump diode laser. In this context an essential component of diode performance is a smooth dependence of output light intensity L on injection current input I over their complete range of operation.
From the earliest days it was apparent that diode lasers were highly susceptible to optical feedback induced by reflections from outside the laser cavity. However, it was soon found that under proper control, such as coupling a portion of the diode output back into the laser cavity in a controlled manner by use of external mirrors or gratings, this initially undesirable property could be used to advantage. Although a complete description of the possible ranges of behavior as a function of current modulation and feedback intensity is extremely complex, stable narrow-linewidth single-mode operation has successfully been obtained in external cavity lasers both in the weak and strong feedback regimes. In both these categories several device configurations have been developed with laser linewidths as low as a few kHz and with continuous wavelength tunings in excess of 100 kHz. However, in an intermediate-intensity feedback regime between these two ranges of stable single mode stability, a lack of coherence develops between the diode cavity field and the reflected field. As a consequence the linewidth dramatically broadens and the laser undergoes transition to a regime referred to in the literature as a ‘coherence collapsed’ state.
If controlled feedback is provided by free-standing mirrors or gratings then additional optical elements are usually required to manipulate and guide the light. A great simplification can be effected in this regard by using for the reflecting element a fiber Bragg grating (FBG) written into the output fiber itself. If the wavelength of maximum grating reflectivity is selected to lay near the peak of diode bandwidth then the Bragg grating locks the diode cavity output to the Bragg wavelength of the grating. This locking results in a reduction of output noise, an increase in side-mode suppression and an increase of laser stability. In principle, an external cavity feedback laser should serve as an efficient high power single mode optical source if operated in the strong feedback regime (for which the front facet of the laser is given an antireflective coating to enable the grating feedback to be comparable to or larger than the front facet feedback). In practice, however, the single mode stability of the laser disappears in dramatic fashion with even modest degrees of modulation of the injection current. The source can be traced to the fact that the refractive index n of the diode material is sensitive to the value of the injection current, primarily (though not solely) via an induced generation of heat and the resulting change of temperature T. Since the magnitude of the temperature dependence of refractive index dn/dT of a typical diode laser semiconductor material is at least an order of magnitude larger that that of the FBG material (silica), the temperature dependence of the laser cavity modes (about 0.1 nm/C) far exceeds that of the Bragg wavelength (about 0.01 nm/C) even if the grating experiences the full current-induced temperature variations. The result is a breakdown of mode locking with current modulation. In single mode strong feedback operation, using antireflective coating, the laser operates with a typical mode separation of about 0.1 nm. For a mode spacing of this order, the above mechanism leads to a separate destabilizing mode-hopping event for about every 1 C of temperature change.
In a typical external cavity diode laser the fiber Bragg grating spliced into the output fiber possesses a reflection spectrum equal to approximately 5-6 spacings of the laser cavity centered, say, at a wavelength λ0. If the distance between the FBG and the diode laser cavity is smaller than the coherence length of the laser without feedback then single mode Fabry-Perot (FP) operation can be stabilized by constructive interference between reflection from the laser front facet and the grating. The pump wavelength is then associated with one of the laser FP modes within the grating reflection spectral range. However, with increasing injection current the temperature of the active region rises and, as a consequence of the large difference in dn/dT between the semiconductor and glass, the laser FP modes redshift relative to λ0 inducing a series of laser wavelength jumps from one FP mode to another. These jumps result in kinks in the L-I curve. An example is shown in FIG. 1 (taken from M. Achtenhagen, S. Mohrdiek, T. Pliska, N. Matuschek, C. C. Harder, and A. Hardy, IEEE Photonics Technology Letters Vol. 13, pp415-417) where the associated power fluctuations are also depicted in a more detail as derivatives (i.e. the efficiency) dL/dI as a function of I.
Kinks of this kind are severely detrimental to laser operation and solution has been sought either by dithering the driving current (M. Ziarri et al. “Stabilization of Laser Sources with Closely-Coupled Optical Reflectors using an Internal Dither Circuit”, U.S. Pat. No. 6,215,809) or by removing the FBG to distances beyond the coherence length of the laser (see, for example, B. F. Ventrudo and G. Rogers, “Fibre-Grating-Stabilized Diode Laser”, U.S. Pat. No. 6,044,093). In the latter case, for which distances are typically of order 50 to 100 cm, the reflection can no longer interfere constructively and the feedback from the FBG breaks the coherence of the diode laser emission. In either case the pump laser undergoes power fluctuations that keep the laser in a transient multimode regime—the regime of so-called coherence collapse (see ‘Reflection Noise in Index-Guided InGaAsP Lasers’ by H. Temkin, N. Anders Olssen, J. H. Abeles, R. A. Logan and M. B. Panish, IEEE J. Quantum Electronics, Vol. 22, pp. 286-293). In this multimode regime the light intensity depends on current continuously, eliminating the L-I kinks of single-mode operation. However, the maximum optical power obtained in this regime is substantially lower than that of single mode operation. A schematic representation of the prior art, as illustrated by an external cavity laser with remote FBG, is shown in FIG. 2. The figure comprises a laser source 203, such as a semiconductor laser diode, optically coupled via front facet 201 to an optical fiber waveguide 206. The laser source 203 has an active region 202, which is pumped by means of a supply current 205 via electrodes 204. A Bragg grating 207, written into the fiber waveguide 206, is positioned at a distance from the diode cavity front facet 201 sufficiently large that no constructive interference can take place. With facet 201 given an antireflective coating, the Bragg grating now effectively forms the front facet of the external cavity laser so formed, and induces operation in a ‘coherence collapsed’ kink free state. This approach to pump diode lasers is widely used. However, it has a number of disadvantages. First, the maximum power of operation is still limited by instability due to kinks and low frequency noise. Second, such a design is not compact, and, additionally, it requires special grating arrangements for different laser designs.
Accordingly, it would be desirable to provide a laser source that overcomes the aforementioned disadvantages.